INSECT DIAPAUSE AND ENVIRONMENT 121 



exceptions is provided by the larva of the Chironomid midge 

 Polypedilum vanderplanki which colonizes temporary rock pools 

 in northern Nigeria. In the absence of water the larvae rapidly 

 become completely dehydrated. Yet, a few minutes after re- 

 placing these wizened objects in water, imbibition begins, 

 pharynx and heart contractions appear and the larva begins to 

 swim and feed. Hinton-' ^ has shown that larvae can be desic- 

 cated and rehydrated repeatedly. When thoroughly dry (con- 

 taining less than 1 % water) they can be heated to 100° for short 

 periods, to 65° for one day and will withstand several hours 

 exposure to absolute ethanol or 77 h in liquid air. There is no 

 measurable respiration. Although capable of resisting dry 

 storage for over five years, they are not immortal; and after 10 

 years over calcium chloride larvae often rehydrate satisfactorily 

 but fail to feed and metamorphose. 



In this example dormancy is directly controlled by the 

 availability of water. In other dormant or diapause states the 

 relationship between the environment and the onset or termina- 

 tion of the dormant state is more indirect. The relevant com- 

 ponent, whether photoperiod, temperature or nutrition, acts on 

 the tissues through an intermediate physiological mechanism, 

 namely the endocrine system. I shall consider the question of 

 receptor structures, the role of hormones in the control of 

 diapause and biochemical events in the tissues in my second 

 lecture. At the moment I wish to concentrate on the types of 

 environmental 'cues' which are utilized by arthropods for 

 controlling their dormant periods and comment briefly on the 

 type of physiological information which can be secured by 

 studying these external agencies. 



INDUCTION OF DIAPAUSE 



A considerable array of factors, differing according to species, 

 are concerned in controlling the onset or induction of diapause. 

 In temperate climates photoperiod is one of the most important. 

 Many insects and mites can be said (by analogy with plants) to 



References p. 128 



